<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <meta http-equiv="X-UA-Compatible" content="ie=edge">
  <title>Document</title>
  <style>
    #canvas {
      position: absolute;
      top: 0;
      right: 0;
      bottom: 0;
      left: 0;
      margin: auto;
      background: rgba(0, 0, 0, .5);
    }
  </style>
</head>
<body>
  <canvas id="canvas" width="860" height="860"></canvas>
  <script src="canvas.js"></script>
</body>
</html>